H 2 O (l) H + (aq) + OH (aq)

Transcription

1 What is ph? Water not only serves as the solvent in solutions of acids and bases, it also plays a role in the formation of the ions. In aqueous solutions of acids and bases, water sometimes acts as an acid and sometimes as a base. You can think of the self ionization of water as an example of water assuming the role of an acid and a base in the same reaction: H 2 O H 2 O H 3 O + + OH equal Pure water contains concentrations of H + and OH ions produced by self ionization. One molecule of water acts as a Bronsted Lowry acid by donating a hydrogen ion/proton to the second water molecule, the Bronsted Lowry. base The 1:1 ratio between the products means that equal numbers of hydronium ions and hydroxide ions are formed. The equation for the equilibrium can be simplified in this way: H 2 O (l) H + (aq) + OH (aq) The double arrow indicates that this is in an. equilibrium Recall the equilibrium constant expression is written by placing the concentrations of the products over the concentration of the. reactants K eq = The concentration of pure water is constant so we write a new equilibrium constant, K w. K w = [H + ] ] (aq) [OH (aq) [ H 2 O (l) ] [H + (aq) ] ] [OH (aq) The constant, K w, is called the ion product constant for water: the value of the equilibrium constant expression for the self ionization of water. This is a special equilibrium constant that applies ONLY to the self ionization of water. Experiments show that in pure water at 298K, [H + ] and [OH ] are both equal to M. Therefore, at 298 K, the value of K w is: K w = [H + ][OH ] = ( M)( M) = 1.0 x No UNITS! 1

2 H 2 O (l) H + (aq) + OH (aq) ALWAYS The product of [H + ] and [OH ] equals at 298 K. This means that if the [H + ] increases, the [OH ] must. decrease Similarly, an increase in the [OH ] causes a in the [H + ]. decrease You can think about these changes in terms of Le Chatelier s principle: When a stress is applied to a system in equilibrium, the equilibrium will shift to relieve the stress. Adding extra hydrogen ions to the self ionization of water at equilibrium is a stress on the system. The system reacts in a way to relieve that stress. The extra hydrogen ions react with the hydroxide ions that are already present in the equilibrium to form more water molecules. Thus, the [OH ] decreases. The following example shows how you can use K w to calculate the concentration of either the hydrogen ion or the hydroxide ion if you know the concentration of the other ion. Example: At 298 K, the [H + ] of an aqueous solution is M. What is the [OH ] in the solution? Is the solution acidic, basic or neutral? Step 1: Analyze the problem Known Unknown [H + ]= 1.0x10 5 M [OH ]=? Kw = 1.0x10 7 Step 2: Solve for the unknown K w = [H + ] [OH ] [OH ] = K w [H + ] = 1.0 x x 10 5 = 1.0 x 10 9 M Step 3: Evaluate the Answer How do we know if the solution is acidic, basic or neutral? [H + ]= 1.0x10 5 M [OH ] = 1.0 x 10 9 M [H + ]>[OH ] Therefore, the solution is acidic. Question #1 2

3 ph and poh As you have probably noticed, concentrations of H + ions are often small numbers expressed in scientific notation. Because these numbers are somewhat hard to work with, chemists adopted an easier was to express [H + ] using a ph scale based on common logarithms. The ph of a solution is the negative logarithm of the hydrogen ion concentration: ph = log [H + ] below At 298 K, acidic solutions have ph values 7. Basic solutions have ph values above 7. Thus, a solution having a ph of 0.0 is a strong acid; a solution having a ph of 14.0 is a strong base. A solution with a ph of 7 is considered. neutral The logarithmic nature of the ph scale means that a change of one ph unit represents a tenfold change in ion concentration! Hence, a solution having a ph of 3.0 has 10 times the [H + ] of a solution with a ph of 4.0. The following figure shows the ph scale and ph values of some common substances: Example: Calculating ph from [H + ] What is the ph of a solution with a [H + ]= 1.0 x M concentration? ph = log [H + ] = log (1.0 x ) = NOTE: There are no units for ph. Values of ph are expressed with as many decimal places as the number of sig. figs. in the [H + ]. Because the concentration of [H + ] had 2 sig. figs, the ph should have 2 numbers after the decimal. Question #2 Using poh Sometimes chemists find it convenient to express the basicity, or, alkalinity of a solution on a poh scale that mirrors the relationship between ph and [H + ]. The poh of a solution is the negative logarithm of the hydroxide ion concentration: poh = log [OH ] 3

4 At 298 K, a solution having a poh less than 7.0 is ; basic a solution having a poh of 7.0 is neutral; and a solution having a poh greater than 7.0 is. acidic As with the ph scale, a change of one poh unit expresses a tenfold change in ion concentration. For example, a solution with a poh of 2.0 has 100 times the hydroxide ion concentration of a solution with a poh of 4.0. A simple relationship between ph and poh makes it easy to calculate either quantity if the other is known: ph + poh = The following figure illustrates the relationship between ph and [H + ] and the relationship between poh and [OH ] at 298 K. Use this diagram as a reference until you become thoroughly familiar with these relationships. Example: Calculating poh and ph from [OH ] An ordinary household ammonia cleaner is an aqueous solution of ammonia gas with a hydroxide ion concentration of M. Calculate the poh and the ph of a typical cleaner at 298 K. poh = log [OH ] = log (4.0 x 10 3 ) 2 sig. figs. = = #'s after the decimal! Solve for ph using the equation: ph + poh = ph = poh = = Question #3 4

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